Search Results (21 total)

The beam-spin asymmetries in the hard exclusive electroproduction of photons on the proton (e→p→epγ) were measured over a wide kinematic range and with high statistical accuracy. These asymmetries result from the interference of the Bethe-Heitler process and of deeply virtual Compton scattering. Over the whole kinematic range (x_B from 0.11 to 0.58, Q² from 1 to 4.8  GeV², -t from 0.09 to 1.8  GeV²), the azimuthal dependence of the asymmetries is compatible with expectations from leading-twist dominance, A≃asin⁡ϕ/(1+ccos⁡ϕ). This extensive set of data can thus be used to constrain significantly the generalized parton distributions of the nucleon in the valence quark sector.

The beam spin asymmetry (BSA) in the exclusive reaction e→p→epπ⁰ was measured with the CEBAF 5.77 GeV polarized electron beam and Large Acceptance Spectrometer (CLAS). The x_B,Q²,t, and ϕ dependences of the π⁰ BSA are presented in the deep inelastic regime. The asymmetries are fitted with a sinϕ function and their amplitudes are extracted. Overall, they are of the order of 0.04–0.11 and roughly independent of t. This is the signature of a nonzero longitudinal-transverse interference. The implications concerning the applicability of a formalism based on generalized parton distributions, as well as the extension of a Regge formalism at high photon virtualities, are discussed.

The present experiment exploits the interference between the deeply virtual Compton scattering (DVCS) and the Bethe-Heitler processes to extract the imaginary part of DVCS amplitudes on the neutron and on the deuteron from the helicity-dependent D(e→,e^′γ)X cross section measured at Q²=1.9  GeV² and x_B=0.36. We extract a linear combination of generalized parton distributions (GPDs) particularly sensitive to E_q, the least constrained GPD. A model dependent constraint on the contribution of the up and down quarks to the nucleon spin is deduced.

We have measured the beam-normal single-spin asymmetry in elastic scattering of transversely polarized 3 GeV electrons from unpolarized protons at Q²=0.15, 0.25  (GeV/c)². The results are inconsistent with calculations solely using the elastic nucleon intermediate state and generally agree with calculations with significant inelastic hadronic intermediate state contributions. A_n provides a direct probe of the imaginary component of the 2γ exchange amplitude, the complete description of which is important in the interpretation of data from precision electron-scattering experiments.

We investigated simultaneously the ¹²C(e,e^′p) and ¹²C(e,e^′pp) reactions at Q²=2  (GeV/c)², x_B=1.2, and in an (e, e^′p) missing-momentum range from 300 to 600  MeV/c. At these kinematics, with a missing momentum greater than the Fermi momentum of nucleons in a nucleus and far from the delta excitation, short-range nucleon-nucleon correlations are predicted to dominate the reaction. For (9.5±2)% of the ¹²C(e,e^′p) events, a recoiling partner proton was observed back-to-back to the ¹²C(e,e^′p) missing-momentum vector, an experimental signature of correlations.

An experiment measuring electroproduction of hypernuclei has been performed in hall A at Jefferson Lab on a ¹²C target. In order to increase counting rates and provide unambiguous kaon identification two superconducting septum magnets and a ring imaging Cherenkov detector were added to the hall A standard equipment. An unprecedented energy resolution of less than 700 keV FWHM has been achieved. Thus, the observed _Λ¹²B spectrum shows for the first time identifiable strength in the core-excited region between the ground-state s-wave Λ peak and the 11 MeV p-wave Λ peak.

We measured the angular dependence of the three recoil-proton polarization components in two-body photodisintegration of the deuteron at a photon energy of 2 GeV. These new data provide a benchmark for calculations based on quantum chromodynamics. Two of the five existing models have made predictions of polarization observables. Both explain the longitudinal polarization transfer satisfactorily. Transverse polarizations are not well described, but suggest isovector dominance.

We present the first measurements of the e→p→epγ cross section in the deeply virtual Compton scattering (DVCS) regime and the valence quark region. The Q² dependence (from 1.5 to 2.3  GeV²) of the helicity-dependent cross section indicates the twist-2 dominance of DVCS, proving that generalized parton distributions (GPDs) are accessible to experiment at moderate Q². The helicity-independent cross section is also measured at Q²=2.3  GeV². We present the first model-independent measurement of linear combinations of GPDs and GPD integrals up to the twist-3 approximation.

We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from ⁴He at an average scattering angle ⟨θ_lab⟩=5.7° and a four-momentum transfer Q²=0.091  GeV². From these data, for the first time, the strange electric form factor of the nucleon G_E^s can be isolated. The measured asymmetry of A_PV=(6.72±0.84_(stat)±0.21_(syst))×10^-6 yields a value of G_E^s=-0.038±0.042_(stat)±0.010_(syst), consistent with zero.

We have measured parity-violating asymmetries in elastic electron-proton scattering over the range of momentum transfers 0.12≤Q²≤1.0  GeV². These asymmetries, arising from interference of the electromagnetic and neutral weak interactions, are sensitive to strange-quark contributions to the currents of the proton. The measurements were made at Jefferson Laboratory using a toroidal spectrometer to detect the recoiling protons from a liquid hydrogen target. The results indicate nonzero, Q² dependent, strange-quark contributions and provide new information beyond that obtained in previous experiments.

We have studied the quasielastic ³He(e,e^′p)²H reaction in perpendicular coplanar kinematics, with the energy and the momentum transferred by the electron fixed at 840 MeV and 1502  MeV/c, respectively. The ³He(e,e^′p)²H cross section was measured for missing momenta up to 1000  MeV/c, while the A_TL asymmetry was extracted for missing momenta up to 660  MeV/c. For missing momenta up to 150  MeV/c, the cross section is described by variational calculations using modern ³He wave functions. For missing momenta from 150 to 750  MeV/c, strong final-state interaction effects are observed. Near 1000  MeV/c, the experimental cross section is more than an order of magnitude larger than predicted by available theories. The A_TL asymmetry displays characteristic features of broken factorization with a structure that is similar to that generated by available models.

Results of the Jefferson Lab Hall A quasielastic ³He(e,e^′p)pn measurements are presented. These measurements were performed at fixed transferred momentum and energy, q=1502  MeV/c and ω=840  MeV, respectively, for missing momenta p_m up to 1  GeV/c and missing energies in the continuum region, up to pion threshold; this kinematic coverage is much more extensive than that of any previous experiment. The cross section data are presented along with the effective momentum density distribution and compared to theoretical models.

We report a virtual Compton scattering study of the proton at low c.m. energies. We have determined the structure functions P_LL-P_TT/ϵ and P_LT, and the electric and magnetic generalized polarizabilities (GPs) α_E(Q²) and β_M(Q²) at momentum transfer Q²=0.92 and 1.76  GeV². The electric GP shows a strong falloff with Q², and its global behavior does not follow a simple dipole form. The magnetic GP shows a rise and then a falloff; this can be interpreted as the dominance of a long-distance diamagnetic pion cloud at low Q², compensated at higher Q² by a paramagnetic contribution from πN intermediate states.

Exclusive electroproduction of π⁰ mesons on protons in the backward hemisphere has been studied at Q²=1.0  GeV² by detecting protons in the forward direction in coincidence with scattered electrons from the 4  GeV electron beam in Jefferson Lab’s Hall A. The data span the range of the total (γ*p) center-of-mass energy W from the pion production threshold to W=2.0  GeV. The differential cross sections σ_T+ϵσ_L, σ_TL, and σ_TT were separated from the azimuthal distribution and are presented together with the MAID and SAID parametrizations.

The differential cross section for electron-induced neutron knockout in the reaction ⁴He(e,e^′n)³He has been measured for the first time with a statistical accuracy of 11%. The experiment was performed in quasielastic kinematics at a momentum transfer of 300  MeV/c and in the missing-momentum range of 25–70  MeV/c. The comparison of the data with theoretical calculations shows an impressive increase of the cross section resulting from final state interaction effects. Specifically , the p-n charge-exchange process dominates the cross section in this kinematical regime.

Tensor polarization observables ( t₂₀, t₂₁, and t₂₂) have been measured in elastic electron-deuteron scattering for six values of momentum transfer between 0.66 and 1.7 (GeV/c)². The experiment was performed at the Jefferson Laboratory in Hall C using the electron High Momentum Spectrometer, a specially designed deuteron magnetic channel and the recoil deuteron polarimeter POLDER. The new data determine to much larger Q² the deuteron charge form factors G_C and G_Q. They are in good agreement with relativistic calculations and disagree with perturbative QCD predictions.

The A(Q²) structure function in elastic electron-deuteron scattering was measured at six momentum transfers Q² between 0.66 and 1.80 (GeV/c)² in Hall C at Jefferson Laboratory. The scattered electrons and recoil deuterons were detected in coincidence, at a fixed deuteron angle of 60.5°. These new precise measurements resolve discrepancies between older sets of data. They put significant constraints on existing models of the deuteron electromagnetic structure, and on the strength of isoscalar meson exchange currents.

The analyzing power A_y0 of the reaction H(p→,d)π⁺ has been measured at a fixed value of the Mandelstam variable u_d=-0.17 GeV² for nine proton energies between 1000 and 1300 MeV. The experiment was performed at SATURNE with the SPES1 spectrometer. The data exhibit structure around sqrt[s]≃2.37 GeV. The origin of this structure could be related to a resonancelike behavior of the ¹S₀P or ¹G₄F partial amplitudes.

A Reply to the Comment by J. Jourdan, I. Sick, and J. Zhao.

The ratio of neutron and proton yields at quasifree kinematics was measured for the reactions ²H(e,e^′n) and ²H(e,e^′p) at momentum transfers Q²  =  0.125, 0.255, 0.417, and 0.605 (GeV/c)², detecting the neutron and the proton simultaneously in the same scintillator array. The neutron detection efficiency was measured in situ with the ¹H(γ,π⁺)n reaction. From this the ratio R of ²H(e,e^′n) and ²H(e,e^′p) cross sections was determined and used to extract the neutron magnetic form factor G_Mⁿ in a model insensitive approach, resulting in an inaccuracy between 2.1% and 3.3% in G_Mⁿ.

We report on the first measurement of tensor polarization transfer coefficients for the excitation of the 12.7 MeV 1⁺ state of ¹²C by (d→,d→) inelastic scattering at 393 MeV. The experiment has been performed at Saturne with a tensor polarimeter. The data have allowed the determination of the spin-flip probability S₁, found to be in good agreement with the approximate signature S_d^y determined previously with a vector polarimeter. The relationships between measured polarizations and analyzing powers are discussed.